1. Field of the Invention
The present invention relates to an apparatus and method for acoustic imaging, which is defined herein to mean electronic reconstruction and display of the size, shape, and internal elastic and viscous properties (e.g., density, acoustic speed, and acoustic energy absorption) of an object or material. More particularly, the present invention relates to an apparatus and method for acoustic imaging using inverse scattering techniques.
2. The Prior Art
It has long been known that acoustic waves in the frequency range of a fraction of a cycle per second up to hundreds of millions of cycles per second and higher can be propagated through many solids and liquids. Acoustic energy waves may be partially reflected and partially transmitted at the interface between two media of different elastic properties. The product of material density and sonic wave velocity is known as the acoustic impedance, and the amount of reflection which occurs at the interface between two media is dependent upon the angle of incidence and the amount of change in the acoustic impedance from one medium to the other. This concept of reflection from layers may be generalized to reflection from small regions of arbitrary shape. If the regions of differing impedance are of the order of a wavelength or smaller, the reflection is no longer specular, but diffuse. In this case, the more general term of scattering is used to include both specular and diffuse reradiation of energy. It is also seen that scattering is produced not only by fluctuations in impedance, but also by fluctuations in speed of sound, compressibility, density, and absorption. The net property of an object which describes this phenomenon is called the scattering potential.
These principles have been used for imaging reflecting bodies within a propagation medium. In terms of scattering theory, the direct or forward scattering problem is concerned with a determination of the scattered energy or fields when the value and distribution of the elastic or electromagnetic properties of the body (i.e., scattering potentia) or the distribution of the particles doing the scattering are known. The inverse scattering problem consists in the use of scattered electromagnetic and/or acoustic waves to determine the internal material properties (i.e., scattering potential) of objects from the information contained in the incident and scattered fields. In other words, as defined herein, acoustic imaging using inverse scattering techniques is intended to mean electronic reconstruction and display of the size, shape, and unique distribution of material elastic and viscous properties of an object scanned with acoustic energy, i.e., reconstruction of that scattering potential which, for a given incident field and for a given wave equation, would replicate a given measurement of the scattered field for any source location.
Acoustic imaging through the use of inverse scattering techniques has been a much studied problem in fields which are as diverse as seismic geophysical surveying, nondestructive testing, sonar, and medical imaging. Such inverse scattering techniques would be of particular interest because of the ability to provide accurate quantitative as well as qualitative image values when using such techniques. However, the use of complete inverse scattering techniques in acoustic imaging is generally considered to be so difficult that it has been common to employ methods which lead merely to approximations rather than actual image values. For example, one approach used in holographic imaging and seismic imaging is to "back propagate" the detector field measurement into the object, usually assuming as a model a homogeneous or a one-dimensional layered distribution of wave propagation speed. The images obtained are images of the source of the scattered fields, and thus only indirectly provide an indication of internal structure of material properties. Many of such techniques are described in the Acoustical Imaging series, volumes 1-13, published by Plenum Press, Inc.
In the case of medical diagnostic imaging, quantitative tissue characterization based on approximate or theoretically incomplete inverse scattering techniques are now being investigated for inclusion on clinical pulse echo scanners, also known as B-scanners. However, tissue characterization using such scanners is based on 180 degree backscattering from structural and statistical properties of tissues and not on determining absolute tissue properties, per se. The statistical properties of tissues, e.g., texture or the spatial Fourier transform, are often correlated with the state of health or disease and are therefore valuable, but they are not easy to measure quantitatively using present incomplete or approximate inverse scattering techniques. thus, while it is possible with the present state of the art to obtain some quantitative information about tissue properties from B-scans, it is not possible to obtain absolute mechanical properties of such tissues.
In summary, prior art apparatus and methods which have been used to date do not take into account such problems as multiple orders of scattering compensation for refraction, frequency dependent effects of density on scattering properties of the object, changes in acoustic absorption based on changes in frequency of the acoustic energy, or boundary value measurements of the incident field, scattered field, and scattering properties of the object. All of these problems may significantly affect image quality. Yet the prior art has largely ignored these problems by using approximations or assumptions which avoid having to account for such problems when reconstructing the image. Accordingly, it would be an important advance in the state of the art to be able to provide acoustic imaging using inverse scattering techniques which provide an image of the actual material properties of an object (and not just the internal fields) without use of perturbation or other drastic approximations, such as the well-known ray optics, single scattering, Born, or Rytov approximations. Such an apparatus and method are described and claimed herein.